1. Field of the Invention
The present invention relates to telecommunications systems and, particularly, to a system and method for integrating Session Initiation Protocol (SIP) agents into an H.323 system.
2. Description of the Related Art
The International Telecommunications Union (ITU) Recommendation H.323 describes a set of devices and protocols for multimedia communication over packet-switched networks. The four main components defined by the specification are clients (also referred to as terminals or endpoints), multipoint control units, gateways (also referred to as an endpoint) and gatekeepers.
A diagram illustrating an exemplary H.323 telecommunications system 100 is shown in FIG. 1. The telecommunications system 100 includes a local area network (LAN) or packet network 101. Coupled to the LAN 101 may be a variety of H.323 terminals 102a, 102b, a multi-point control unit (MCU) 104, an H.323 gateway 106, an H.323 gatekeeper 108, a LAN server 112 and a plurality of other devices such as personal computers (not shown). The H.323 terminals 102a, 102b support H.245 control signaling for negotiation of media channel usage, Q.931 (H.225.0) for call signaling and call setup, H.225.0 Registration, Admission, and Status (RAS), and RTP/RTCP for sequencing audio and video packets.
The gatekeeper 108 is responsible for bandwidth management and address resolution. Each endpoint has a unique user ID or alias to represent an H.323 user. Each endpoint can have multiple aliases to represent multiple users. Each alias, before it can make or receive calls, must register with the gatekeeper 108. When an alias makes a call to another alias, the gatekeeper 108 verifies whether the called party is a valid alias and whether network bandwidth is available. If so, the gatekeeper 108 accepts the call request.
Typically, H.225.0 call signaling and H.245 call control signaling are routed through the gatekeeper 108, while the media channels (i.e., audio, data and/or video) are routed directly between the endpoints. For example, to place a call between two clients, the calling client sends a message to the gatekeeper 108, which resolves the address of the receiving party and sends the appropriate signaling messages to the caller and receiver. Once the signaling and control channels have been established, the endpoints establish the media channels. The media channel passes directly between the endpoints. The H.225.0 signaling channel and H.245 call control channel are also used to terminate the call.
While the H.323 Recommendation is a widely accepted standard for IP telephony, it is a very sophisticated protocol and it is relatively difficult to implement a simple IP telephone based on H.323. The Session Initiation Protocol (SIP) is an emerging application layer signaling protocol that is relatively easy to implement. Thus, it is desirable to integrate SIP protocol devices into an existing H.323 based system for simple IP phone implementation.
These disadvantages in the prior art are overcome in large part by a system and method according to the present invention. An SIP-H.323 gateway is provided, which functions as a multi-alias H.323 endpoint with one alias representing one SIP agent. The SIP-H.323 gateway processes the call signaling conversions and stores and passes media port information to the signaling parties, so that the existence of SIP agents is transparent to the H.323 endpoints and the features are transparently available to the H.323 endpoints.
A SIP-H.323 gateway according to the present invention includes a SIP protocol process engine, an H.323 stack, and also maintains a SIP alias table which has an alias for each SIP agent. In operation, the H.323 stack is initialized to include all the possible SIP agents as aliases and stores the SIP agent information in the SIP alias table. The SIP-H.323 gateway performs signaling conversion for any SIP agent whose information is stored in the table.